This invention relates to an improved process for controlling the temperature in the regeneration zone in a fluid catalytic cracking process. In particular, it is related to a method for maintaining the temperature in the fluidized dense catalyst phase of the regenerator of a fluid catalytic cracking unit (FCCU) having a single fluidized dense catalyst phase wherein coke-contaminated fluidizable catalytic cracking catalyst is contacted with an oxygen-containing regeneration gas in order to obtain a regenerated catalyst having a low carbon content.
The fluidized catalytic cracking of hydrocarbons is well-known in the prior art and may be accomplished using a variety of continuous cyclic processes which employ fluidized solids techniques. In such fluid catalytic cracking processes hydrocarbons are converted under conditions such that substantial portions of a hydrocarbon feed are transformed into desirable products such as gasolne, liquified petroleum gas, alkylation feedstocks and middle distillate blending stocks with concomitant by-product formation of an undesirable nature, such as gas and coke. When substantial amounts of coke deposition occur, reduction in catalyst activity and, particularly, catalyst selectively results thereby deterring hydrocarbon conversion, reducing gasoline production and simultaneously increasing the production of less desired products. To overcome such catalyst deactivation through coke deposition, the partially deactivated coke-contaminated catalyst (hereinafter referred to as spent catalyst) is normally withdrawn from the reaction zone and passed to a stripping zone wherein entrained and absorbed hydrocarbons are initially displaced from the catalyst by means of stripping medium such as steam. The steam and hydrocarbons are removed and the stripped catalyst is passed to a regeneration zone where it is contacted with an oxygen-containing gas to effect combustion of at least a portion of the coke and thereby regenerate the catalyst. Thereafter, the regenerated catalyst is reintroduced to the reaction zone and therein contacted with additional hydrocarbons.
Generally, regeneration processes provide a regeneration zone wherein the spent catalyst is contacted with sufficient oxygen-containing regeneration gas at an elevated temperature to effect combustion of the coke deposits from the catalyst. Most common of the regeneration processes are those wherein the contacting is effected in a fluidized dense catalyst phase in a lower portion of the regeneration zone constituted by passing the oxygen-containing regeneration gas upwardly through the regeneration zone. The space above the fluidized dense catalyst phase contains partially spent regeneration gases and catalyst entrained by the upward flowing regeneration gas. This portion of the regeneration zone is generally referred to as the dilute catalyst phase. The catalyst entrained in the dilute catalyst phase is recovered by gas solid separation cyclones located in the upper portions of the regeneration zone and is returned to the fluidized dense catalyst phase. Flue gas comprising carbon monoxide, other by-product gases obtained from the combustion of the coke deposits, inert gases such as nitrogen and any unconverted oxygen is recovered from the upper portion of the regeneration zone and a catalyst of reduced carbon content is recovered from a lower portion of the regeneration zone.
In the regeneration of catalytic cracking catalyst, particularly high activity molecular sieve type cracking catalysts, it is desirable to burn a substantial amount of coke from the catalyst such that the residual carbon content of the regenerated catalyst is very low. A carbon-on-regenerated-catalyst content of about 0.15 weight percent or less is desirable. Cracking catalysts with such a reduced carbon content enable higher conversion levels within the reaction zone of the FCC unit and improved selectivity to gasoline and other desirable hydrocarbon products.
In the regeneration of catalytic cracking catalyst it is also desirable to operate the regeneration zone under conditions such that the flue gas leaving the regeneration zone have a carbon monoxide concentration of approximately 500 ppm or less so that the flue gas may be discharged into the atmosphere without additional treatment.
In order to obtain low carbon-on-regenerated-catalyst contents of about 0.15 wt.% or less, and a regeneration flue gas having a low carbon monoxide content, it is necessary to operate the fluidized dense catalyst phase of the regeneration zone at a temperature of from about 1275.degree.F. to about 1450.degree.F. and provide oxygen-containing regeneration gas in an amount sufficient to effect combustion of the coke to carbon dioxide and to provide from about 1 to about 10 mol% oxygen in the flue gas in order to reduce the carbon monoxide concentration in the flue gas to the levels herein indicated.
It frequently occurs in FCCU operations, that a particular desired operating condition will result in a spent catalyst being recovered from the reaction zone, which has a coke concentration that is not sufficient to provide the necessary heat, when burned in the regeneration zone, to maintain the fluidized dense catalyst phase at the controlled operating temperature. For example, a FCCU unit may be operated such that the regeneration zone is maintained at relatively high temperatures necessary to effect the substantially complete combustion of coke from the spent catalyst and to provide a regeneration flue gas with a carbon monoxide concentration of 500 ppm or less. Because of the high temperature and activity of the regenerated catalyst, it may be necessary to decrease the catalyst-oil ratio (lb. cat./lb. oil) in the transport reaction zone to maintain the desired conversion level. Under such conditions with high activity catalyst, such as zeolitic molecular sieves, the coke laydown on the catalyst in the reaction zone may not be great enough to provide sufficient heat, upon combustion in the regeneration zone, to maintain the operating temperature of the fluidized dense catalyst phase at the controlled temperature.
Also, when a FCCU unit is operated with pure riser cracking, the coke concentration of the spent catalyst is generally lower than in conventional FCCU operations. Therefore, it may be necessary to add other combustible materials to the regeneration zone in order to maintain the desired operating temperature. Still another operating condition wherein additional heat must be added to the regeneration zone, involves the cracking of light hydrocarbon feedstocks, such as naphtha, in which the coke laydown on the catalyst in the reaction zone is lower than in conventional FCCU operations.
Known methods for providing this additional heat in the regeneration zone generally involve injecting torch oil or other combustible materials by means of a nozzle or a plurality of nozzles directly into the fluidized dense catalyst phase of the regeneration zone. Such methods are unsatisfactory in that hot spots are created in the fluidized dense catalyst phase around the area where the torch oil or other combustible materials is injected into the regeneration zone. The temperatures within these hot spots generally exceed about 1500.degree.F. and may exceed about 1800.degree.F. or higher. Such high temperatures are deleterious to the catalyst and result in a permanent loss of catalytic activity, thus necessitating an inordinately high rate of catalyst addition or replacement to the process in order to maintain a desired level of catalytic activity in the hydrocarbon reaction zone.
Whenever a regenerator is operated within the range of operating conditions herein described, it is important to control the temperature of the fluidized dense catalyst phase in the regenerator in order to maintain the desired operating conditions in the reaction zone and in order to avoid uncontrolled afterburning in the dilute catalyst phase of the regeneration zone.
By after-burning is meant the further oxidation of carbon monoxide to carbon dioxide in the dilute catalyst phase. Whenever after-burning occurs in the dilute catalyst phase, it is generally accompanied by a substantial increase in the temperature due to the large quantities of heat liberated. In such circumstances the dilute phase temperature may exceed about 1500.degree.F. and, in severe cases, may increase to about 1800.degree.F. or higher. Such high temperatures in the dilute catalyst phase are deleterious to the entrained catalyst present in the dilute catalyst phase and result in a permanent loss of catalytic activity, thus necessitating an inordinately high rate of catalyst addition or replacement to the process in order to maintain a desired level of catalytic activity in the hydrocarbon reaction zone. Such high temperatures are additionally undesirable because of the damage which may result to the mechanical components of the regeneration zone, particularly to cyclone separators employed to separate the entrained catalyst from the flue gas.
It is known that commonly employed catalytic cracking catalysts such as amorphous silica-alumina, silica-alumina zeolitic molecular sieves, silica-alumina zeolitic molecular sieves ion-exchanged with divalent metal ions, rare earth metal ions, etc., and mixtures thereof, are adversely affected by exposure to excessively high temperatures. At temperatures of approximately 1500.degree.F. and higher, the structure of such catalytic cracking catalyst undergo physical change, usually observeable as a reduction in the surface area with resulting substantial decrease in catalytic activity. Consequently, it is desirable to maintain the temperatures within the regeneration zone at levels below which there is any substantial physical damage to the catalyst.